Calcium aluminates, through their hydraulic and binder properties, make it possible to manufacture cements or concretes that have many qualities. The aluminous cements indeed resist aggressive agents and high temperatures well. They are the source of many technical products such as special mortars, refractory concretes, etc. They can also be used as a mineral reagent associated with other components. They are as such used in various industries, such as the refractory industry, construction chemistry, metallurgical flows (trapping of impurities of metals in fusion), or in the industry of pipes and sanitation networks.
Calcium aluminates can have different mineralogical phases such as 3CaO.Al2O3(C3A in cement notation), CaO.Al2O3(CA), CaO.2Al2O3(CA2), CaO.6Al2O3(CA6) or 12CaO.7Al2O3(C12A7). These mineralogical phases, which express the structure on an atomic scale as well as the chemical composition of the calcium aluminates, influence the final properties, for example the reactivity, of said calcium aluminates.
Furthermore, the final properties, and in particular the reactive properties of the calcium aluminate based products are partially based on the quantity of alumina (Al2O3) and/or of aluminium (Al) and of calcium oxide (or lime CaO) and/or of calcium (Ca) contained in the calcium aluminate. A calcium aluminate is often qualified by its Al/Ca weight ratio, namely the ratio between the total mass of aluminium and the total mass of calcium contained in the calcium aluminate.
Currently, calcium aluminates are primarily produced using two high-temperature methods, namely according to a method by sintering or according to a method by melting, in cement kilns such as sintering furnaces, rotary flame furnaces, vertical melting furnaces, or electric melting furnaces.
For example document FR2291162 discloses a method for producing calcium aluminates by sintering which consists in calcinating, i.e. in heating in the solid state raw materials sources of calcium, for example lime CaO, and raw materials sources of aluminium, for example alumina Al2O3, in a rotary flame furnace, at a temperature between 1,400° C. and 1,600° C.
Generally, a rotary furnace is comprised of a tube, slightly inclined, covered on its inside face with refractory bricks, with a flame being arranged at the bottommost end of the tube. The sources of calcium and of aluminium are then introduced into the furnace by the highest end. They are then generally heated to a temperature between 1,400 and 1,600 degrees Celsius (° C.) for a duration of about 30 minutes before being discharged into the bottom portion, in the vicinity of the flame.
Such a method by sintering consists of a surface reaction between the powdery raw materials that react together without passing through a generalised liquid state.
According to the method described in this document FR2291162, the raw materials used must have a granulometry smaller than 208 micrometres in order to allow for their calcination. The clinker obtained has more than 80% of mineralogical phase CA.
It is therefore necessary to very finely grind the raw materials, which is expensive and constraining.
It is moreover known in document FR1013973, a method for producing calcium aluminates by melting according to which raw materials sources of calcium and of aluminium are heated until liquid state in a rotary flame furnace, at temperatures in the vicinity of 1,430° C. to 1,450° C.
The raw materials sources of calcium (CaCO3 limestone) and of aluminium (ferruginous bauxite, a mineral rock rich in alumina and containing iron, silica and other compounds in variable quantities) used in such a method are firstly ground very finely in order to pass through the screen no. 4900, then mixed and compacted in order to be formed into briquettes. The briquettes generally have an average dimension between 15 millimetres (mm) and 20 centimetres (cm).
According to the method described in this document FR1012973, the method is discontinuous and sequenced: first the briquettes of raw material are loaded, these briquettes of raw material are then heated by rotating the furnace slowly until a uniform melted mass is obtained, and the melted mass is drained via a casting tube. In practice, the melted mass is gathered immediately after having reached the melting temperatures of the raw materials.
On the other hand document DE2116495 discloses a method for producing calcium sulfo-aluminates by melting in an electric furnace.
The raw materials used in this method are a source of alumina which average granulometry (i.e. the maximum value of the granulometric distribution) is smaller than 5 mm and a source of calcium oxide such as lime which average granulometry is one to ten times greater than that of alumina.
The method described in this document DE2116495 is a sequenced method, a sequence corresponding to the load of raw materials, to the melting thereof, and to a partial unloading of the melted mass.
Thus, these known methods by melting are implemented in furnaces wherein it is necessary to operate in a sequenced and discontinuous manner.
Furthermore, known melting furnaces have several operating modes (for example an operating mode for loading and an operating mode for unloading) and their operation requires many passages from one operating mode to another.
It is finally known that a vertical melting furnace can be used to carry out methods by melting.
This vertical melting furnace has a vertical portion which height can reach about ten metres and a globally horizontal portion from which the liquid mass of calcium aluminates obtained is gathered.
More particularly, blocks of limestone and of bauxite are loaded via an opening of an upper zone of the furnace in the vertical portion of the melting furnace and they are heated via a flame arranged in a lower zone of the furnace. The flame heats the blocks to a temperature in the vicinity of 1,500° C. in order to melt them and form a liquid mass which is directly gathered through a casting hole.
During the method, combustion gases form and flow against the pathway of the blocks. They are exhausted through a chimney located in the upper zone of the furnace, in its vertical portion. These combustion gases, having a temperature greater than 1,500° C., circulate between the blocks and preheat them.
Before being put into contact with the flame, the blocks of raw materials thus undergo a drying, then a dehydration and a decarbonation due to the combustion gases rising through the vertical portion of the melting furnace.
Such a method imposes the use of raw materials in blocks, excluding fine particles which would cause blockage and damages to the vertical portion of this melting furnace.
Thus, the known methods for the production of calcium aluminates by melting or by sintering are all constraining in terms of the granulometry of the raw materials used (fine grinding of the raw materials; grinding then compaction on the form of briquettes; specific granulometric ratio between the lime and the alumina; or the use of blocks of bauxite).
In particular, in the case of the method by melting in the vertical melting furnace, the blocks of bauxite on the market are less and less available. Furthermore, during the extraction, the yield of production of the blocks of bauxite is low. Indeed, for 100 tonnes of ore extracted, only 10 tonnes of crude bauxite are obtained which themselves contain 8 tonnes of fine particles that cannot be used in the method by melting in the vertical furnace, and 2 tonnes of bauxite in blocks that can be used, along with 90 tonnes of sterile items that cannot be used by the industries.